Description: (Applicant's Description) With the Support of the NCDDG, the Experimental Therapeutics Group at MSKCC developed a core expertise in the evaluation of telomerase activity and telomere measurements in clinical samples from patients with a variety of malignancies. In addition, in vivo a number of different leukemia and solid xenograft tumor models were established for evaluating telomerase inhibitor candidates from collaborators in the Screening and Medicinal Chemistry Groups at Geron. The role of Experimental Therapeutics Group is to develop and evaluate lead telomerase candidates in in viva tumor models. As lead candidates are developed that overcome hurdles of potency and selectivity, they will progress to cell-based assays. Depending on the efficacy of lead compounds in a spectrum of human tumor cell lines, the telomerase inhibitor will be evaluated in an appropriate in viva model